High fatigue bolt thread



March 18, 1969 E. F. GOWEN, JR, ETAL 3,

HIGH FATIGUE BOLT THREAD Filed June 5, 1967 T u 1 w 7 Twl T T I L ET I Im I Z M. m o E 5 .M f 7 6 U 7 III III lll xlflv A n R n Y O {4 w 5 HAS/CP/ INVENTOR. Wf M L BY 0 ATTOR/VIXS United States Patent 3,433,117 HIGHFATIGUE BOLT THREAD Edward F. Gowen, Jr., Churchville, Raymond Ollis,Jr.,

Oreland, and Richard A. Walker, Warrington, Pa., assignors to StandardPressed Steel Co., J enkintown, Pa., a corporation of PennsylvaniaContinuation-impart of applications Ser. No. 285,980, June 6, 1963, andSer. No. 494,150, Oct. 8, 1965. This application June 5, 1967, Ser. No.643,617 The portion of the term of the patent subsequent to June 6,1984, has been disclaimed U.S. Cl. 85-1 4 Claims Int. Cl. F16b 35/00,27/00, 29/00 ABSTRACT OF THE DISCLOSURE A screw thread member whereinthe threads are formed by rolling in such a manner that the resultingthread form has an asymmetric thread profile with the pressure flankangle being 32-37 and the non-pressure flank angle being 30. Inaddition, the threads have a uniform negative lead correction of 0.001to 0.007 in./in. of thread.

This invention relates to an externally threaded member that isespecially useful in the thread system described in our co-peudingapplication Ser. No. 494,150 filed Oct. 8, 1965, now Patent No.3,323,402, as a continuation of our then co-pending application Ser.:No. 285,980 filed June 6, 1963, now abandoned; and this application isa continuation-in-part of the aforesaid applications.

The thread system specified in the aforesaid applications is one thatincreases the fatigue life of threaded elements by effecting certainmodification as to the form and relative arrangement of the threads ofthe internal and externally threaded members or elements; viz., theapplications are concerned with a screw thread system including aninternally threaded member and an externally threaded member whoseflanks, crests, and roots are formed by rolling, said system ischaracterized in that- (a) The pressure flank of the internal threadfrom its root and at least to about the vicinity of its pitch line isdisengaged from the pressure flank of the external thread and thepressure flank of the internal thread is in contact with the pressureflank of the external thread in the vicinity of the pitch line when thesystem is loaded.

(b) The internal threads have, when in an unloaded condition, aprogressively longer lead than the lead of the unloaded externalthreads, the lead differential being sufficient to compensate for theshortening of the lead of the internal threads and the elongation of theexternal threads which occurs on loading of the thread system andthereby provide for a more uniform distribution of stress concentrationat the roots of the engaged external threads, and,

(c) Said thread system being further characterized in that the truepitch diameters of the internal and external threads are corrected tocause the virtual pitch diameters to approach the basic pitch diameterand thereby prevent an interference fit when in an unloaded condition.

The present aspect of the invention is concerned with providingexternally threaded screw thread members which when used with aconventional internally threaded member, will provide a thread system ofthe general type contemplated in the aforesaid applications and alsoprovide the benefits achieved thereby.

The externally threaded screw thread elements contemplated by thisaspect of the invention are characterized in that the thread has anasymmetric thread profile, the flank angle of the pressure flank beingabout 32 to 37 (preferably 35) and the flank angle of the non-pressureflank is about 30; the threads have a uniform negative lead correctionof 0.001 to 0.007 inch per inch of thread; and in that the threads(i.e., the roots, crests and flanks) are formed by rolling butpreferably without packing the crests in the rolling operation.

The external thread form and formulae (maximum material condition) ofthreads of the preferred asymmetric 65 including angle embodiment ofthis invention is illustrated in FIGURE 1. This embodiment is intendedto be used with conventional symmetrical 60 included angle matinginternal threads (preferably internal threads meeting militaryspecification MH.S-'8879).

As can be seen in FIGURE 1, the profile of the thread is asymmetric inform, the flank angle f of the pressure flank 10 is 35 and the flankangle f of the non-pressure flank 11 is 30", thus giving an includedangle of 65. It will be understood however, that the benefits of thisinvention can be obtained with asymmetric included angle configurationsof about 65 i about 2; in such arrangements the non-pressure flankshould have a more or less conventional flank angle of 30 and thepressure flank shall carry the principal flank angle modification.

The external threads have a uniform negative lead correction of 0.001 to0.007 inch per inch of thread and in the preferred asymmetric 65included angle embodiment, the lead correction shall be uniformly minus0.0025 in./ in. 2 0.0010.

In the preferred asymmetric 65 included angle embodiment of theinvention, the root of the asymmetric thread is modified from that of asymmetrical 60 included angle thread in that the root is formed by aradius R that blends with the non-pressure flank 11 at a gageinterference point 13 of 68% to of the unified basic thread depth h, andthe same radius blending with the pressure flank 10 at a gageinterference point 12 of 70% to 77% of the unified basic thread depth h.In both cases, the measurements are from the truncated crest 14 of theasymmetric thread; the crest shall be truncated from the sharp crest ofthe fundamental (60) triangle an amount equal to 0.17771H where H is thedepth of the fundamental (60) triangle. [H and h are dimensions definedin Handbook H2-8 (1957), Screw Thread Standard for Federal Services,National Bureau of Standards] When the 65 included angle asymmetric,lead Corrected external thread is used in combination with aconventional symmetrical 60 mating internal thread, the virtual pitchdiameter of the external threads must be corrected so that the virtualpitch diameters of the internal and external threads approach the basicpitch diameter which is located at 0.34179H from the truncated crest andtherefore the unloaded system will be free of interference fit.

As was pointed out in the parent applications, in the preferredembodiment, the flanks, roots and crests of the external thread areformed by a one step rolling operation. In this connection, machiningthe correction onto a conventional thread form will induce surfacestresses and necessitate a retempering operation to remove the stresses.Further, rolling permits a greater freedom of formation and enableshigher strength materials (e.g., up to 260,000 p.s.i.) to be threaded.In rolling the thread, if the rolling conditions are such that therolling operation fully forms the thread but does not compact or packthe material in the vicinity of the crest still greater enhancement offatigue life is obtained. If the crest is compacted or coined, thefatigue life of the member will be adversely afiected to a verysignificant degree.

When an externally threaded member as described above (a rolled threadwhose form is modified so as to provide the asymmetric 65 iabout 2included angle and the prescribed negative lead correction) is used incombination with a conventional symmetrical 60 included angle internallythreaded member, the fatigue life enhancement of the system will begreater than the fatigue life changes obtained by the combined additiveeflects of each of the individual thread form modifications and threadrolling applied alone to the externally threaded member.

We claim:

1. A screw thread member having external threads characterized in that-(a) the flank angle of the pressure flank is 3237;

(b) the flank angle of the non-pressure flank is 30;

(c) the threads have a uniform negative lead correction of 0.001 to0.007 in./in. of thread and in that (d) the threads are formed byrolling.

2. A screw thread member according to claim 1 wherein the threads areformed by rolling but without packing the crests.

3. A screw thread member according to claim 1 characterized in that- (1)the flank angle of the pressure flank is 35;

(2) the flank angle of the non-pressure flank is 30, and

(3) the threads have a uniform negative lead correction of 0.0025- 0.001in./in. of thread.

4. An externally threaded member according to claim 3 and furthercharacterized in that the root of the thread is a radius blending withthe non-pressure flank at a gage interference point of 68% to 75% of theunified basic thread depth and with the pressure flank at a gageinterference point of 70% to 77% the unified basic thread depth.

References Cited UNITED STATES PATENTS 2,842,180 7/1958 Brown et a1.151-22 3,050,318 8/1962 Van Der Wissel 151-22 3,069,961 12/1962 Baubles85-1 3,129,963 4/1964 Robbins 85-46 3,323,402 6/1967 Gowen et al 85-46FOREIGN PATENTS 194,042 1/ 1965 Sweden.

MARION PARSONS, JR., Primary Examiner.

US. Cl. X.R. 85-46

